mpdparser: support BaseURL for SegmentURL using mediaRange
mpdparser: support BaseURL for SegmentURL using mediaRange

This is used in big_buck_bunny_CappedVBR_Multirate_4s_0_AudioVideo_MainProfile.mpd

Orange-OpenSource/gstdashdemux,Orange-OpenSource/gstdashdemux,Orange-OpenSource/gstdashdemux